Processes for patterning semiconductor wafers typically rely on lithographic transfer of a desired image from a thin-film of radiation-sensitive resist material. The process entails the formation of a sacrificial layer, the “resist”, which is photo-lithographically patterned. Generally these resists are referred to as “photoresists”.
The patterning of the resist involves several steps, including exposing the resist to a selected light source through a suitable mask to record a latent image of the mask and then developing and removing selected regions of the resist. For a “positive” resist, the exposed regions are transformed to make such regions selectively removable; while for a “negative” resist, the unexposed regions are more readily removable.
The pattern can be transferred into surface texture in the wafer by etching with a reactive gas using the remaining, patterned resist as a protective masking layer. Alternatively, when a wafer is “masked” by the resist pattern, it can be processed to form active electronic devices and circuits by deposition of conductive or semiconductive materials or by implantation of dopants.
Materials used in single layer photoresists for optical lithography should meet several objectives. Low optical density at the exposure wavelength and resistance to image transfer processes, such as plasma etching, are two important objectives to be met by such a photoresist material. Shorter wavelengths of radiation permit greater resolution. The most common wavelengths currently used in semiconductor lithography are 365 nm, 248 nm and more recently 193 nm. The desire for narrower linewidths and greater resolution has fueled an interest in photoresist materials that can be patterned by even shorter wavelengths of light.
In the field of microfabrication, the processing size has become more and more minute in order to achieve higher integration. In recent years, development of lithographic processes enabling stable microfabrication with a line width of 0.5 microns, more preferably 0.2 microns or less, has been of keen interest.
However, it is difficult to form fine patterns with high accuracy using conventional methods which utilize visible rays (wavelength: 700-400 nm) or near ultraviolet rays (wavelength: 400-300 nm). To address this problem, lithographic processes using radiation with a shorter wavelength (wavelength: 300 nm or less) have been developed. Such shorter wavelength radiation can achieve a wider range of depth of focus and is effective for ensuring design rules with minimum dimensions.
Examples of short wavelength radiation, deep ultraviolet rays, such as those generated from a KrF excimer laser (wavelength: 248 nm), or an ArF excimer laser (wavelength: 193 nm) can be utilized as well as X-rays such as synchrotron radiation, charged particle rays such as electron beams and the like. However, the polymeric materials used with such processes are limiting in terms of composition, chemical resistance, transparency to DUV and physical characteristics.
There is a need in the art for novel polymeric materials that are transparent for use in DUV, to allow penetration of activating light, and that are robust enough to withstand further processing conditions.